In general, in a wireless communication system that uses a terminal device such as a cell phone, the terminal device can access a base station apparatus by using an access method called a random access when the terminal device starts to communicate with the base station apparatus. In the case of a random access, the terminal device transmits an identifier called a preamble to the base station apparatus by using a random access channel (RACH) to establish synchronization between the terminal device and the base station apparatus.
The random access includes two kinds of random accesses of a terminal device-led random access and a network-led random access in which the network includes a base station apparatus. In a terminal device-led random access, a terminal device randomly selects one preamble from a utilizable preamble group that is previously defined and transmits the selected preamble to a base station apparatus. Therefore, a plurality of terminal devices may transmit the same preamble at the same timing in some cases to cause conflict between terminal device-led random accesses. For this reason, a terminal device-led random access is also referred to as a contention based random access. Because a process for canceling a conflict is required when the conflict occurs, the terminal device-led random access causes delayed synchronization establishment between the terminal device and the base station apparatus.
On the other hand, in the case of a network-led random access, a terminal device previously requests a base station apparatus to assign a preamble and transmits the preamble assigned by the base station apparatus to the base station apparatus. Therefore, because the conflict of a preamble transmitted by the terminal device is not caused, a network-led random access is also referred to as a non-contention based random access.
Although a wireless communication system can employ only one of a contention based random access and a non-contention based random access, the wireless communication system can also combine both random accesses to realize a random access. In other words, a part of an utilizable preamble group can be used as a preamble for a contention based random access and the rest can be used as a preamble for a non-contention based random access. Such a random access is realized, for example, when a terminal device performs a handover or when the synchronization establishment of an uplink from a terminal device to a base station apparatus is required. This technique has been known as disclosed in, for example, Japanese Laid-open Patent Publication No. 2008-219852.
As described above, a non-contention based random access can prevent the delay of synchronization establishment between a terminal device and a base station apparatus without causing the conflict of a preamble. However, because a non-contention based random access restricts the number of preambles that can be assigned to terminal devices by each base station apparatus, there is a problem in that a preamble is short when many terminal devices request the assignment of a preamble. In other words, when a preamble is once assigned to a terminal device, this preamble is occupied by the terminal device for dozens of milliseconds to several seconds and thus can not be used by other terminal devices. Therefore, when the number of terminal devices that execute a non-contention based random access around the same time increases, a preamble cannot be assigned to a part of the terminal devices.
Particularly, in the case of a combined random access of a contention based random access and a non-contention based random access, a preamble for a non-contention based random access may be short when the assignment of a preamble is performed in the order by which the assignment request of a preamble arrives at a base station apparatus. A terminal device to which a preamble for a non-contention based random access is not assigned due to the shortage of a preamble performs a contention based random access. At this time, for example, when a preamble for a non-contention based random access is not assigned to a terminal device that requires a constant QoS (Quality of Service), the required QoS may not be achieved. In other words, there is a problem in that a required bandwidth is not secured for a terminal device that requires QoS higher than another terminal device.
It is specifically explained about the problem with reference to FIG. 22. In FIG. 22, preambles #1 to #16 are preambles for a non-contention based random access and preambles #17 to #64 are preambles for a contention based random access. Moreover, terminal devices indicated by a slant line in the diagram are guaranteed bit-rate QoS terminal devices and terminal devices indicated by a white space in the diagram are best-effort QoS terminal devices. In other words, terminal devices indicated by a slant line in the diagram require QoS higher than terminal devices indicated by a white space in the drawing.
In FIG. 22, preambles are assigned to terminal devices that request the assignment of a preamble on a first-come-first-served basis. For this reason, when the preambles #1 to #16 are all assigned, even a QoS terminal device of which the bandwidth is assured should perform a contention based random access by using any of the preambles #17 to #64 after the assignment. According to a contention based random access, because the conflict of a preamble selected by a terminal device may occur, a communication start between the terminal device and the base station apparatus is delayed. As a result, a bandwidth balanced with the QoS of the terminal device cannot be assured.